One of the major challenges facing many countries around the world is to provide clean water for various human activities (drinking, agricultural and industrial) and to cover the needs of the population growth. Although the needs for clean water are a critical issue in developing countries, the developed countries are also suffering from the continuous shortage in freshwater resources due to water pollution from industrial processes and urbanization. Subsequently, the needs for wastewater treatment in developed countries have become a pressing environmental issue due to the increasing requirements in effluent quality regulations.
For example, one of an important environmental issue is the biological nutrient removal (BNR). Although many wastewater treatment plants are capable of biological nutrient removal, the regulations are changing in some areas to take wastewater treatment to a higher level requiring enhanced nutrient removal (ENR). Enhanced nutrient removal most often requires phosphorus levels to be less than 0.3 mg/L and total nitrogen of 3 mg/L or less. The lowering of nitrogen and phosphorus levels to increase the quality of the wastewater effluent, (i.e. enhanced nutrient removal (ENR)), is accomplished through the use of advanced wastewater technologies.
Consequently, the currently available conventional wastewater treatment technologies are no longer responding to new standards, and there is an increasing desire for the development of innovative, more effective and inexpensive techniques for wastewater treatment.
On the other hand, the continuous pollution of the receiving bodies highlights the trend to further manage treated wastewaters by changing the total water recycle approach, which promotes ecological sustainability by recognizing the treated wastewater as a water source instead of a wasted medium. This new view may lead to a reduction of the demand for water from existing water resources.
To fulfill the above requirements, an attention on advanced wastewater treatment has become an international hot issue during the last years. Membrane processes belong to this group and attract a high degree of attention from researchers.
In the last decades of the 20th century, membrane technology (MT), especially pressure driven membrane group, has been given great attention and it has been proven to be a promising technology for the purification of drinking water, for wastewater treatment and for reuse applications.
Membrane technology was firstly limited to the tertiary treatment stage for disinfection and polishing of the effluent from the secondary treatment. However, in 1969, membrane technology was integrated directly with the activated sludge process to form one technology called membrane bioreactor technology (MBR). The idea of MBR technology was first developed to replace the secondary clarifier in the activated sludge process (ASP) to overcome the settling difficulties associated with the process and to get a good quality effluent. Although MBR solved many problems associated in ASP, fouling of the membrane is till the major factor in decreasing the wide-spread use of the process. Originally, the membrane module was utilized outside the reactor, but further development of the process made the membrane module integrated inside the bioreactor. This new configuration was called a submerged membrane bioreactor (SMBR). Consequently, the MBR technology has two configurations in terms of process operation: external and submerged membrane bioreactors. The SMBR overcomes the limits of the external one by the lower power cost; therefore, most of the recent studies focus on the development of this type of configuration.
The SMBR as a second generation MBR technology can lead to a revolution in wastewater treatment methods if the fouling problem can be reduced. It has been reported that SMBR would be a good alternative for wastewater treatment plants in comparison with ASP if the fouling problem is finally eliminated.
Accordingly, for SMBR systems to be commercially competitive in comparison with ASP, further development of the process is required to decrease the fouling rate of the membrane. In this domain, many studies have been conducted to reduce this problem. In general, there are many methods to reduce fouling in SMBR technology. Those methods can be grouped by three distinct approaches: cleaning the membrane unit, optimizing the operating parameters and improving the wastewater characteristics. Membrane cleaning is the common approach used in most of the SMBR applications. Cleaning the membrane is achieved physically by backwashing the permeate or back flushing using a high flow rate stream of air. This technique results in an increase in the operating costs and the high flow rate of air may cause damage for the membrane module. On the long run, membrane can be washed chemically to recover the membrane permeability.
Optimization the operating parameters includes the selection of the best operating conditions in terms of aeration, sludge retention time (SRT), hydraulic retention time (HRT) and MLSS concentration in the bioreactor to minimize the fouling on the membrane.
Improving the characteristics of the treated wastewater has been proven an effective approach in reducing the fouling in SMBR applications. This approach includes the addition of chemical coagulant as alum and iron salts to increase the floc size of the MLSS solution or adding adsorptive materials like high concentration of powdered activated carbon material.
Increasing the size of the MLSS flocs solution by coagulation has been proven to be an effective method. However, the addition of chemicals to the wastewater may cause side effects by producing byproducts or increasing the volume of sludge in the reactor. Another technology to create coagulation inside the system, suggested by the current inventors, is introducing electrokinetic processes to biological process. In this case, one of the electrokinetic processes is electro-coagulation (EC). EC has been proved to be a good method for coagulation in wastewater. In comparison with the chemical coagulation (CC) processes, electrocoagulation (EC) has many advantages: no liquid chemical is added, alkalinity is not consumed, and the EC process requires less coagulant and produces less sludge.